Cyanoacrylates have been known for a long time. Their use particularly as instant adhesives has been well documented with a huge amount of research directed towards identifying cyanoacrylate compositions for specific end use applications. The use of CAs extends across the fields of mechanical, electrical, medical, automotive, consumer industries and many other fields. A very great number of applications in these fields are well known and are not described in detail here.
One observed phenomenon with cyanoacrylate formulations is that they tend to cure to form relatively brittle polymeric materials. This is an undesirable property for certain applications where a degree of flexibility in the polymeric material is desired. Such applications include bonding flexible materials where a degree of flexibility in the bond to match the flexibility of the material is desired. Such flexible materials include textile materials, flexible plastics, elastomeric materials, films, and other flexible materials such as rubber. It is also desirable to have a flexible polymeric material in applications where the polymeric material may be subjected to varying forces in its end-use application. For example if the polymeric material has bonded together two substrates, the substrates may not remain in an undisturbed condition but may be subject to external forces, for example where the substrates form part of a moving object, or part of a stationary object which is subjected to one or more continuous or occasional forces from other moving objects. It is desirable that cyanoacrylate compositions could be used to form flexible polymeric materials of any desired shape or for any desired use such as in coating, producing films, layers etc.
Accordingly much effort has been directed to providing cyanoacrylate formulations which cure to form a flexible polymeric material. One of the approaches to overcoming the brittleness of polymerised cyanoacrylate adhesives has been to plasticise the composition. In the literature a distinction has been made between internal and external plasticisation. Internal plasticisation in a cyanoacrylate polymer has been described as being obtainable by the use of monomer mixtures. The use of mixtures of cyanoacrylate monomers is thought to result in a more flexible polymeric material when the monomer mixture is cured. Indeed one review by G. H. Millet entitled “Structural Adhesives Chemistry and Technology”, Plenum Press, New York (1996) adopts this internal and external plasticisation terminology and reports certain manufacturers selling mixed monomer adhesives for elastomer or plastic bonding. A second approach has been to incorporate what are termed as “external plasticisers” in cyanoacrylate compositions. External plasticisation is achieved by incorporating in the cyanoacrylate composition a component which imparts the desired flexibility to the polymerised material. The flexibility is generally described as being achieved as a compromise as deleterious effects to other desirable properties such as slower cure speeds, lower bond strength and often opacity of cured material may result due to the presence of external plasticiser.
Many external plasticisers show only an initial effect so that when the composition is first polymerised the polymeric material formed is relatively flexible. With many plasticisers however ageing of the polymeric material (often a bond) reveals that the flexibility initially imparted to the material by incorporation of the plasticiser is lost as time passes. Accordingly with many of the known plasticisers the polymeric material becomes brittle over a relatively short period, for example a number of weeks, and the initial desired flexibility is lost. Furthermore it has been noted that using internal plasticisation may suffer from the same drawback.
An alternative approach to overcoming the flexibility deficiency in the polymerised material has been to develop high molecular weight cyanoacrylate monomers. These polymers are thought to polymerise to form polymers which exhibit greater internal flexibility. Such monomers however are relatively expensive to manufacture and have not generally received wide-spread acceptance for large scale applications.
U.S. Pat. No. 2,776,232 (Shearer and Coover) refers to overcoming the identified problem of brittleness in polymeric materials prepared from cyanoacrylate monomer mixtures of at least two different α-cyanoacrylates. The mixed cyanoacrylate monomer compositions are described as being formed by individually de-polymerising the desired monomer from its polymer or de-polymerising a mixed polymer to give a mixed monomer. The '232 patent describes including a plasticiser such as tricresyl phosphate to give a low melting and readily depolymerised polymer so as to allow ease of obtenation of the monomer(s). Other uses of plasticiser are not mentioned. The '232 regards that the mixture of the polymers in the composition result in adhesive bonds which are considerably more flexible after polymerization than are the bonds of homopolymers of α-CAs. No data is supplied to support the assertion of increased flexibility in the polymeric bond or the polymer itself. It is not believed that the compositions proposed in the '232 patent sufficiently solve the brittleness problem associated with the polymerised cyanoacrylate material to allow compositions exhibiting desired properties to be made reliably. In particular it is thought that any flexiblisation achieved will not be maintained over time.
U.S. Pat. No. 2,784,215 (Joyner) refers to compositions including alkoxy alkyl or thioalkoxyalkyl esters of α-cyanoacrylic acid. The monomers proposed by the '215 patent are also suggested as being miscible with alkyl α-cyanoacrylates such as methyl α-cyanoacrylate to give compositions which form high strength bonds with alleged excellent ageing properties and flexibility characteristics which are desired as being better than those demonstrated by the alkyl α-cyanoacrylate bonds and not as good as those properties exhibited by the monomers which are the subject of the '215 patent.
U.S. Pat. No. 4,364,876 (Kimura et al) is directed to, what was at the time, a novel 2-cyanoacrylate monomer. The '876 patent also refers to the use of the monomers provided with more conventional 2-cyanoacrylate monomers. The '876 patent refers generally to the addition of a plasticiser which may result in a flexible cured product. This flexible cured product is described as being obtainable by adding dioctyl phthalate, dibutyl phthalate, trioctyl trimellitate, dioctyl adipate, dioctyl glutarate in an amount of 0–50% by weight based on the weight of the 2-cyanoacrylate.
U.S. Pat. No. 2,784,127 (Joyner and Coover) is directed to the problem of overcoming brittleness found in the polymer of cyanoacrylate monomers. The '127 patent is particularly directed to addressing the problem of objectionable bond failure after ageing. The patent refers to adhesive compositions composed of monomeric α-cyanoacrylate esters and refers to the possibility of mixtures of the monomeric α-cyanoacrylate esters being used. The patent refers to using ester plasticisers of defined types in an amount from 1–20% by weight and in particular from about 1–5% by weight.
While Millet refers to internal and external plasticisation, the '127 patent makes no distinction between internal and external plasticisation. The provision of a small amount of butyl cyanoacetate to flexibilise the polymeric material is employed in Examples 3, 4 and 8 in addition to the cyanoacrylate monomer component. Examples 5–7 and 9–11 are directed to the use of a monomeric cyanoacrylate together with an external plasticiser. The '127 patent does not supply any quantitative data to support the contention that a more flexible bond is achieved. It is not believed that the '127 patent teaches how to reliably formulate a cyanoacrylate composition which yields a polymeric material which demonstrates the type of flexibility described in the opening passage of the '127 patent as being desirable when polymerised, and indeed after a period of ageing of at least a few weeks.
U.S. Pat. No. 3,699,127 (O'Sullivan et al) is directed to novel plasticisers which are organic ethers, in particular aryl or diaryl ethers, where the aromatic ring of each aryl group is directly bonded to the ethereal oxygen. The plasticisers are referred to as useful in a range of between 0.2% and 15% by weight of the adhesive composition. The desired range is between 05% and about 10% by weight. The '127 patent states that above about 15% by weight the presence of the plasticisers can affect adversely the strength of the cured bond.
Further description of plasticisers compositions can be found in UK Patent No. 1 529 105. The UK '105 patent is directed to the problem of providing an adhesive formulation which combines the rapid bonding associated with cyanoacrylate with reduced and controlled bond strength along with the ability to be readily debonded when desired. The composition proposed in the UK '105 patent is an adhesive composition having at least one polymerisable ester of an α-cyanoacrylate acid, from 20% to 60% by weight of the composition of a plasticiser which is miscible with the ester and from 0.015% to 0.15% by weight of the composition of a carboxylic acid which is soluble in the ester. While mention of mixtures of monomeric α-cyanoacrylate esters is made, it is concluded that preferably a single ester is used. The preferred range for the plasticiser is 30% to 40% by weight.
U.S. Pat. No. 3,961,966 (Brinkmann et al) relates to cyanoacrylate compositions comprising plasticisers which are monocyclic or bicyclic lactones having from 3 to 20 carbon atoms and having from 4 to 7 members in the lactone ring.
A further cyanoacrylate-based adhesive composition which is described as having reduced adhesion to the skin is described in U.S. Pat. No. 4,444,933 (Columbus et al). The composition contains a 2-cyanoacrylate ester, a vinyl chloride/vinyl acetate copolymer and optionally plasticiser. A similar disclosure appears in UK Patent No. 2 268 503 B which again relates to cyanoacrylate adhesive compositions having reduced adhesion to the skin. The composition includes a 2-cyanoacrylate, an anionic polymerization accelerator which is least one of an aliphatic alcohol and various aliphatic carboxylic acid esters.
U.S. Pat. No. 5,716,607 (Byram et al) discloses a method for inhibition of late radiation induced skin damage during treatment with ionising radiation. The '607 patent is concerned with the provision of a biocompatible polymeric layer over the surface of the skin to be exposed to the radiation. Cyanoacrylates are preferred monomers for formation of the polymeric layer with n-butyl cyanoacrylate being the preferred and only exemplified monomeric material employed. Addition of a biocompatible plasticiser is also disclosed.
WO 99/18950 discloses mixed alkyl cyanoacrylate compositions which are for topical application onto intact or broken skin and which exhibit sufficient flexibility on curing to allow the integrity of the polymerised material to remain while bonded to the skin. The document describes in general the desirability of non-inclusion of plasticisers to impart the desired flexibility, and aims instead to provide a mixture of CA esters which when polymerised form a polymeric material with the desired flexibility characteristics. In particular the document refers to the addition of a sufficient amount of a C10–C12 alkyl cyanoacrylate ester to a C1–C8 alkyl cyanoacrylate ester so as to form a flexible CA polymer on mammalian skin without the need for a plasticiser. While the document describes the desirability to avoid the addition of plasticisers a number of biocompatible plasticisers are referenced. Exemplified are compositions containing n-butyl and n-decyl cyanoacrylate. Plasticiser (dioctylphthalate) is used in a composition together with n-butyl cyanoacrylate as a comparative formulation. Referenced in WO 99/18950 is U.S. Pat. No. 5,480,935 (Greff et al) which describes cyanoacrylate compositions comprising at least one C4–C10 alkyl cyanoacrylate, a biocompatible plasticiser and SO2 as a polymerisation inhibitor for application to the skin. Examples of biocompatible plasticiser are given.
While many broad disclosures relating to the potential use of two or more cyanoacrylate esters to “internally” flexibilise the resultant polymeric material when the monomer mixture is polymerised and attempts have been made to provide “external plasticisers” which can suitably flexibilise the resistant polymeric material, no specific proposal has been taught or suggested which would identify adhesive compositions which more regularly and more reliably achieve the desired result. Accordingly it is desirable to provide an adhesive composition based on a cyanoacrylate which may be cured to a polymeric material which demonstrates a desired flexibility.
A further desirable result not addressed to any great extent in those documents discussed above is a clear polymerised material. Many end users have a preference for a cyanoacrylate composition which is clear (transparent) and colourless and also prefer to find that the resultant polymeric material is clear (transparent) and colourless also. This is particularly desirable if transparent materials are to be bonded. It is also desirable where excess adhesive for example adhesive squeezed from a gap between substrates or applied on surfaces should not be visible.
In view of the desired properties discussed above, and in view of the previous various attempts to address some of the deficiencies of cyanoacrylate adhesives, it would be desirable to provide cyanoacrylate compositions which cure to a polymeric material of desired flexibility both initially and on ageing, which do not have significant deterioration of other desired properties and which are colourless pre- and post-cure.